This invention relates to a system for assembling a Coriolis flow meter having a flow tube having a straight configuration. More particularly, this invention relates to a system of assembly that joins components made of metals having dissimilar properties in a manner that reduces the amount of stress applied to the structure during assembly. Still more particularly, this invention relates to a system of assembly for enclosing a flow tube made of a first metal in a casing made of a dissimilar metal and attaching at least two points on each end of the flow tube to the casing.
It is known to use Coriolis effect mass flowmeters to measure mass flow and other information of materials flowing through a pipeline as disclosed in U.S. Pat. No. 4,491,025 issued to J. E. Smith, et al. of Jan. 1, 1985 and U.S. Pat. No. Re. 31,450 to J. E. Smith of Feb. 11, 1982. These flowmeters have one or more flow tubes of a curved or a straight configuration. Each flow tube configuration in a Coriolis mass flowmeter has a set of natural vibration modes, which may be of a simple bending, torsional, radial, or coupled type. Each flow tube is driven to oscillate at resonance in one of these natural modes. The natural vibration modes of the vibrating, material filled systems are defined in part by the combined mass of the flow tubes and the material within the flow tubes. Material flows into the flowmeter from a connected pipeline on the inlet side of the flowmeter. The material is then directed through the flow tube or flow tubes and exits the flowmeter to a pipeline connected on the outlet side.
A driver applies a vibrational force to the flow tube. The force causes the flow tube to oscillate. When there is no material flowing through the flowmeter, all points along a flow tube oscillate with an identical phase. As a material begins to flow through the flow tube, Coriolis accelerations cause each point along the flow tube to have a different phase with respect to other points along the flow tube. The phase on the inlet side of the flow tube lags the driver, while the phase on the outlet side leads the driver. Sensors at two different points on the flow tube produce sinusoidal signals representative of the motion of the flow tube at the two points. A phase difference of the two signals received from the sensors is calculated in units of time. The phase difference between the two sensor signals is proportional to the mass flow rate of the material flowing through the flow tube or flow tubes.
Coriolis flowmeters having a straight flow tube have several advantages over flowmeters having two flow tubes of a curved design. One advantage of straight tube flow meter is that single flow tube has a larger diameter which provides a flow path of a greater volume. The flow path having a greater volume reduces plugging, lowers the pressure drop of flowing material entering the meter, and facilitates the cleaning of the flow tubes. A second advantage is that the flow tube is straight. The straight flow tube facilitates cleaning of the flow tube and allows the flow tube to be self draining when oriented correctly.
However, the design of a Coriolis flowmeter having a straight flow tube presents many problems in the manufacture of such a flowmeter. A first problem is that the flow tube must be attached to the casing enclosing the flow tube for structural support. One particular problem with affixing a straight flow tube to a casing is that the flow tube and different components of the casing are made of different metals. Ideally, all of the components of a flowmeter would be made out of the same material and joining the various components would simplified. However, the cost of forming components from metals, such as titanium, Hastelloy, or zirconium, dictates that some components be made of less expensive materials. Typically, components such as the casing that do not have contact with the process material are made of less expensive metals than the metal used to form the flow tube. Sometimes, the components made of an inferior metal may then be covered with a veneer made of another material to provide a sanitary surface and to protect against corrosion of the casing.
The different metals have dissimilar properties which may make it difficult to join the metals by conventional methods such as welding. For example, a flow tube may be made of Titanium while an end of a casing is made of stainless steel. The thermal coefficient of Titanium is 5.3 microin/in xc2x0 F. and the thermal coefficient of stainless steel is 9.6 microin/in xc2x0 F. The disparity between thermal coefficients of expansion make it desirable to use rapid and localized heating to braze elements to limit the effects of the disparity.
A second problem in the connection of straight flow tube to the casing is the electronic components connected to the flow tube. Electronic components include a driver, pick-off sensors, and temperature sensors. These electronic components must be installed on the flow tube and balance bar prior to joining the flow tube and housing. If high temperature joining processes are then used to join the flow tube and casing, the electronic components may be destroyed or damaged by the high temperature.
Any assembly system that solves the above problems must be economical. If the assembly system increases the cost of manufacture, the flowmeters may become too expensive to manufacture and thus cost prohibitive to sell.
The above and other problems are solved and an advance in the art is made by the assembly system of this invention. The assembly system of the present invention is a low cost method for assembling a straight tube Coriolis flowmeter. The system of this invention brazes components in a localized manner to reduce the effects of thermal expansion and to prevent damage to electrical equipment.
In the assembly system of this invention, a Coriolis flowmeter having a straight flow tube is assembled in the following manner to form a flow tube assembly. The process begins by joining a straight flow tube made of a first metal to a balance bar that is oriented substantially parallel to a longitudinal axis of the straight flow tube and that encloses a portion of said straight flow tube. In a preferred embodiment, the balance bar is made of a metal having substantially the same properties as the first metal of the flow tube and the ends of the balance bar are vacuum brazed to the flow tube.
After the flow tube and balance bar are joined to form a flow tube assembly, a drive system and sensors are installed onto the flow tube assembly. The flow tube assembly is then inserted into the casing. The casing is made of a second metal that has dissimilar properties to the first metal. Each end of the flow tube assembly is then joined to at least two points inside the casing using rapid and localized heating. Each point where the flow tube assembly is joined to the casing must be facilitated by a dissimilar metal.
An end of the flow tube assembly is connected to one of the at least two points in the following manner. Case connects are affixed to opposing ends of said straight flow tube and said balance bar. These case connects may be vacuum brazed on to the flow tube assembly at the same time the balance bar is joined to the flow tube. Alternatively, the case connects may be joined to the flow tube assembly after the straight flow tube and the balance bar are joined. Case connects are members that protrude out from the balance bar and flow tube. The case connects are made of a metal having properties substantially similar to both the first and the second metal. This allows the case connect to be brazed to the flow tube assembly and to be welded to the casing. In a preferred embodiment, the case connects are vacuum brazed to the ends of the flow tube and balance bar.
After the flow tube assembly is inserted into the casing, the case connects are affixed to inserts on an interior surface of the casing. The inserts are protruding ledges at opposing ends of the casing. The inserts are made of a metal that has properties that are substantially similar to the metal of the case connect. In a preferred embodiment, the case connect and inserts are welded together by conventional methods.
Also after the flow tube assembly is inserted into the casing, a driver, sensors, and temperature sensors on the flow tube and balance bar are connected to wires that feed through the casing. The wires are connected through the aperture in the casing.
In a preferred embodiment, a second point of attachment for each end of the flow tube is with a case end. The case end is an end covering that is mated with the opposing openings of the aperture. An aperture through the case end allows the flow tube to protrude through the case end. The flow tube is connected to the case end after the flow tube and balance bar have been connected to the casing.
In a preferred embodiment, the flow tube is connected to the case connect in the following manner. After the case connects are welded to the inserts in the casing, each case end is inserted over an end of the flow tube and affixed to an end of the casing over an end of the aperture in the case end. One manner of affixing a case end to an end of the casing is by welding the case end to the casing. Welding is used in the preferred embodiment because the case end is made of stainless steel and the casing is made of Carbon steel.
After the case end is connected to the casing, the case and flow tube are joined. In a preferred embodiment, the case end and flow tube are joined in the following manner. First, a brazing material is placed inside the case end on a lip around the inside circumference of the case end. The case end is then inserted over the flow tube and tacked in place on the casing. The case end is then welded to the casing. The flow tube is then affixed to the case by induction brazing the lip of the case end and the flow tube.
Alternatively, a transition piece is affixed to the flow tube to extend out from the flow tube. The transition piece is made of metal having properties substantially similar to said first metal and the second metal. The transition piece is vacuum brazed to the flow tube after the balance bar has been affixed. The transition piece is then welded into place after the case end is connected to the casing.
After the flow tube assembly is connected to the second of the two points inside the casing, the access openings through the casing are sealed. The access opening are openings through the casing that allow access to components of the flow tube assembly for adjustments after installation into the casing.
A pneumatic test is performed after the access openings are sealed. The pneumatic test pressure rates the casing. After the pneumatic test are complete, a pneumatic opening is sealed. The pneumatic opening is an opening that allows pneumatic test equipment to be inserted into the housing.
The casing is then enclosed in a veneer. The veneer is a covering that provides a sanitary outer surface for the casing. In the preferred embodiment, the veneer is stainless steel and is affixed in the following manner. The veneer is wrapped around the casing. The sides of the wrapped veneer are then welded together by a longitudinal weld. The circumferential ends of the veneer are then affixed by an orbital weld to stainless steel case ends.
A final step in a preferred embodiment is to affix process connections to opposing ends of the flowmeter. The process connections are formed of the first metal of the flow tube and the metal from which the case end is made. The process connection are inserted over the ends of the flow tube and then welded to the case ends of the casing. The first metal in the process connections encloses at least a portion of an aperture through the process connection. The flow tube is welded to the first metal enclosing the portion of the aperture after the process connections are joined to the case ends.